Mattis said he was confident that "it will not spiral out of control," but he warned that the Syrian regime of President Bashar al-Assad will "pay a very, very stiff price" if chemical weapons are used again.

"Right now, the purpose of the attack was singular," Mattis said. The intent of the Tomahawk strikes was to deter the regime of President Bashar al-Assad from further chemical attacks and not to send a message to the Russian military backing the regime, Mattis said.

Earlier, White House officials said there was evidence that Syria used the nerve agent Sarin in an April 4 attack on the northwestern Syrian town of Khan Sheikhoun that reportedly killed at least 89 and injured hundreds.

The officials said Russia either knew, or should have known, beforehand, of the attack and charged that Moscow was engaged in a coverup to protect its Syrian ally.

Mattis said he could not address the level of Russian foreknowledge of the chemical attacks and instead stressed that "Our military policy in Syria has not changed" as the result of the cruise missile strikes on the Shayrat airfield north of Damascus last Thursday.

The main goal of the U.S. military in Syria was the elimination of the threat from the Islamic State of Iraq, which was now focused on working with partnered local forces to take the ISIS stronghold of Raqqa in northeastern Syria, Mattis said.

"This other effort (the missile strikes) came up in the midst of this and had to be addressed," said Mattis "It's not a harbinger of some change in our military campaign" against ISIS, he said.

"There is a limit, I think, to what we can do" in Syria, he said, adding "I'm confident the Russians will act in their own best interests" and avoid confronting the U.S.

"We are focused on defeating ISIS," said Army Gen. Joseph Votel, the commander of U.S. Central Command, who joined Mattis at a late-afternoon Pentagon news conference. However, he acknowledged that U.S. troops in Syria had adopted a more defensive posture as a precaution against a Russian response.

Mattis' remarks contrasted with the more aggressive stance against Moscow taken earlier by White House officials, who charged that Russia was involved in a coverup of the Syrian chemical attack and suggested that Russia also may have known of the planning for the Khan Sheikhoun strike.

A senior White House official speaking on background said, "At this time, there is no U.S. intelligence community consensus that Russia had foreknowledge of the Syrian chemical attack."

However, three other White House officials, also speaking on background, said at a later briefing that Russian troops at the Shayrat airbase north of Damascus could hardly have missed the Syrian preparations.

Either way, "The Russian and Syrian narratives are false" about what happened on April 4 at Khan Sheikhoun in Syria's Idlib governorate, a senior White House official said.

"It's a clear pattern of deflecting blame," the official said in reference to Russian and Syrian charges, renewed Tuesday by Russian President Vladimir Putin, that a conventional airstrike likely hit a rebel storage area for chemical weapons.

The White House officials said that the strike was carried out by a Syrian Sukhoi-22 fighter that was tracked by U.S. intelligence taking off and returning to the Shayrat airbase. The munition dropped by the Su-22 landed in the middle of a street in Khan Sheikhoun, rather than on a building that might have stored chemical agents, the officials said.

One of the officials said U.S. intelligence has obtained physiological samples from victims of the April 4 attack that "are very consistent with nerve agent and Sarin. The physiological samples are consistent with Sarin exposure" and the victims "don't have other wounds or injuries consistent with conventional attack," the official said.

Russian troops who fly attack helicopters out of Shayrat were warned to stay clear of the area before the U.S. destroyers Ross and Porter launched 59 Tomahawk cruise missiles at the airbase in response to Khan Sheikhhoun airstrike, according to the Pentagon. Votel said at the Pentagon briefing Tuesday that 57 of the 59 Tomahawks hit their targets at Shayrat.

Syrian officials associated with the chemical weapons programs of Syrian President Bashar al-Assad were at Shayrat in March and were there again on April 4 before the Su-22 took off, the White House officials said.

Both Secretary of State Rex Tillerson and Army Lt. Gen. H.R. McMaster, said over the weekend that the Russians either knew beforehand of the Syrian chemical attack or were incompetent in not knowing, given their close cooperation with Assad's forces.

Tillerson was arriving in Moscow Tuesday for talks with his Russian counterpart, Foreign Minister Sergei Lavrov, that were originally intended to focus on a possible peace settlement to Syria's six-year-old civil war that could eventually lead to Assad stepping down. The Russians have said that Tillerson would not be meeting with Putin.

At a news conference in Moscow with Italian President Sergio Mattarella on Tuesday, Putin repeated the claim that the chemical attack was the result of a strike on a rebel storage area and said that opposition forces were preparing additional chemical attacks.

"We have information that a similar provocation is being prepared," Putin said. He said another chemical attack could come "in other parts of Syria including in the southern Damascus suburbs where they are planning to again plant some substance and accuse the Syrian authorities" of using chemical weapons.

Other Russian officials mocked the U.S. charges of a chemical attack by Syria, noting that the U.S. claimed Iraq had weapons of mass destruction to justify the 2003 U.S. invasion. No weapons of mass destruction were found.

On Monday, Col. John J. Thomas, a spokesman for U.S. Central Command, said in a briefing to the Pentagon that the U.S. has taken extra defensive precautions in Syria in case of possible retaliation against American forces for the cruise missile attack.

Thomas said that the increased emphasis on defensive measures to protect U.S. troops on the ground in Syria led to a slight and temporary decline in offensive U.S. airstrikes against the Islamic State of Iraq and Syria, the Associated Press reported.